This invention is related to a polyarylene polyether (PAPE) oligomer which is thermally crosslinkable through its vinyl end groups. Such oligomers are disclosed in U.S. Pat. No. 4,562,243, the disclosure of which is incorporated by reference thereto as if fully set forth herein. Many of these oligomers form crosslinked networks which are highly resistant to thermal degradation, some at temperatures in excess of 200.degree. C. The styryl terminated oligomers are of particular interest because they cure rapidly.
The crosslinked oligomers are especially suitable as matrix resins in high performance composites but tend to lack requisite toughness and to be somewhat brittle in applications where high impact resistance is essential, though the styryl terminated PAPE matrix is not as brittle as the highly cross-linked polyvinylbenzyl ethers disclosed by Steiner in U.S. Pat. No. 4,116,936. The problem was to find a way to modify a PAPE oligomer so that it could serve as a cocureable toughening agent when blended with the vinyl-terminated PAPE oligomer.
This invention is therefore more particularly related to the improvement of the toughness and impact of cross-linked matrices of PAPEs formed from vinyl terminated polethers of dihydroxybenzene, dihydroxynaphthalene and diphenols, each referred to herein as a dihydric phenol (DHP), including the corresponding sulfur (thio) compounds, each referred to as a dihydric thiophenol (DHTP). The parent polyethers have a number average molecular weight Mn less than about 10,000, hence termed oligomers. Oligomeric polyethers of DHPs and DHTPs contain at least three aromatic rings, which may have inert substituents, each ring linked to another through an O, Si, C or S atom. By "inert substituents" I refer to substituents which do not react so as to interfere with a hydrosilylation (or "hydrosilation") reaction of the PAPE oligomer which reaction yields siloxane block copolymers. Such DHP and DHTP oligomers are terminated at each end with a phenol or thiophenol group respectively, which group may also have inert substituents.
PAPE is used to designate both a polyarylene polyether and a polyarylene polythioether.
In linear block copolymers, it is well known that segment structure and length play an important role in determining morphology and that certain block structures can produce impact resistance. Though it is possible for siloxane blocks to enhance the toughness of a polymer, the detailed structural requirements of all the components necessary to achieve desired toughness, with its associated morphology, must be understood and experimentally verified. Moreover, it is equally well known that the behavior of block copolymers cannot be predicted from the behavior of random copolymers; and even among block copolymers, we know of no way to predict the behavior of an ABA triblock from that of an ABAB alternating block. Neither do we know of a way to predict how cross-linking influences toughening by a particular block copolymer. It is only after these considerations that we can try to experimentally determine what specific parameters of structure and segment length might provide the desired improvement.
It was found that short PDMS blocks (shorter than Mn=5000) of each alternating block chain are miscible with [4,4'-isopropylidene diphenol]-[4,4'-diphenylsulfone], or, bisphenol A-diphenylsulfone ("BPA-DPS" for brevity) blocks while long PDMS blocks are immiscible and aggregate into separate microdomains. This miscibility pertains to segments within each chain (intrachain), and between chains (interchain). In particular, with PDMS blocks having Mn above 5000, there was extreme microincompatibility of the BPA-DPS and PDMS segments. This extreme incompatibility at relatively short block lengths of Mn=5000 and above, was attributed to the widely different solubility parameters of the BPA-DPS (10.6) and PDMS (7.3) segments.
The effect of block lengths in a particular alternating block copolymer has been studied in an article titled "Morphology and Impact Resistance in Linear and Crosslinked Block Copolymers", by A. Noshay, et al;., Jour. Poly. Sci.: Polymer Symposium 60 87-95 (1977). One alternating block copolymer studied comprised from 3 to about 15 alternating blocks of BPA-DPS (diphenyl sulfone) copolymer with blocks of poly(dimethtylsiloxane) (PDMS). The alternating block copolymer was derived from OH-terminated BPA-DPS by reaction with dimethylamino-terminated PDMS.
Since extreme incompatibility was desirable for toughening, and this was not obtained with block lengths below Mn=5000 (which they found to be miscible), it was obvious that block lengths above Mn=5000 with an alternating block structure, were dictated. It was thus quite fortuitous that we decided to explore the effect of short block lengths, shorter than Mn=5000. Such a short block length, particularly for the PDMS segment, turned out to be the key to obtaining better impact resistance and toughness through microphase separation of the rubbery domains upon curing. In addition, we thought there was a chance we could eliminate the effects of intrachain miscibility of PDMS segments in an alternating block copolymer, if we made a triblock, though the problem of tailoring the lengths of each segment still remained.
From an examination of a crosslinked block copolymer model system of epoxy-anhydride/caprolactone blocks alternating with linear polycaprolactone blocks, Noshay et al concluded that qualitatively, the same features which govern the behavior of uncrosslinked BPA-DPS-PDMS block copolymers also govern that of crosslinked block copolymers. But no suggestion was made as to how to predict such features, and in particular, how to crosslink a BPA-DPSPDMS block copolymer. Nor was there any consideration of the kinetics of the crosslinking reaction which could exert severe constraints against the formation of desired microphase separated morphology when compared with uncrosslinked copolymer.
We know of no teaching as to the applicability of a hydrosilation reaction to provide the proper triblock segment structure for any reason, and particularly to eliminate the intrachain miscibility effects which require the use of relatively long (Mn of 5000 or more) chain segments. Nor do we know of any teaching to suggest a desirable chain length for a crosslinkable PAPE-PDMS-PAPE triblock with vinyl end groups, which when blended with PAPE containing vinyl end groups will form a thermally cocurable matrix with improved impact resistance and toughness.
Various studies have been reported in articles titled "Synthesis of Poly(Sulphone-b-Siloxane)s" by D. Gagnebien et al Eur. Polym. J. Vol.21, No.3, pp 280-308 (1985) to characterize the structure of the block copolymers formed using a hydrosilation reaction to introduce a polysiloxane segment, but there is no teaching as to what the effect of the introduction of a single polysiloxane segment in a chain might be, nor the effect of crosslinking the OH-terminated polysulfone with an oligomer having epoxy chain ends, assuming this could have been accomplished.
The triblock oligomers of this invention were the end result of several syntheses each of which tried to find an effective modification of a PAPE with vinyl end groups to improve its toughness and impact resistance without substantially decreasing the upper glass transition temperature (Tg) of the cured resin, and its resistance to thermal degradation.